1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-VII-5 411
We compare turbulence properties at the extreme values of
the q scan, qa = 2.8 and qa = 0.6. Near the separatrix the
frequency spectra are quite similar at the two q values (with
the rms field falling more than two orders of magnitude from 2°
kHz to 400 kHz). In both cases the radial profiles of Br
(measured from r = 6 cm to the edge) resemble the radial
profiles of the equilibrium quantities (current density or
pressure), being more peaked at high qa than at low qa.
The measured radial correlation within the plasma appears
independent of q. At both q values, the radial coherence length
(e-folding distance of the two-point coherence) decreases with
frequency from about 50 cm at 15 kHz to about one cm at 400
kHz. At low q the correlation length is slightly shorter than
at high q.
The poloidal coherence length, L , is measured with two
poloidally separated coils. One can infer the spread in
poloidal mode numbers, Am, which results in the poloidal decay
of the coherence, via the estimate Am = irr/Lp. At high q, Am
increases with frequency from 2 to 15, whereas at low q, Am
increases from 1 to 5. Qualitatively similar results are
obtained from inferences of m from the radial fall-off of §r in
the vacuum-like scrape-off region. The larger number of
poloidal modes at low qa might account for the enhanced
fluctuation level.
Since L does not vary with radius (and is similar to
values measured in larger tokamaks) the poloidal scalelength of
the fluctuations might not be linked to the minor
circumference, as are most MHD instabilities (characterized by
fixed m values). Perhaps the modes are tied to an intensive
parameter such as gyroradius or resistive layer width.
The overall similarity of the turbulence over the range
0.6 < qa